602 GEOLOGICAL HISTORY. 



(f.) But there are a few breaks of extraordinary character, deserving 

 special consideration. The first Vertebrates, Fishes, start off suddenly 

 in the Upper Silurian ; and no trace of connecting links with Mollusk 

 or Articulate has been found. The Ascidian has been put forward as 

 the origin ; but no intermediate forms between the Ascidian and Ver- 

 tebrate exist among fossils ; and, moreover, as Verrill has observed, after 

 a thorough study of the tribe, the alleged relation to the Vertebrates is 

 without the slightest foundation in their structure. The modern Am- 

 phioxus, — a very small fish without a brain, — has been made to fill 

 the gap. But, although seemingly fitted for the place, it may be only 

 degradational, and of comparatively modern development. The rocks 

 have given us no hint as to its existence in Silurian times, or that of any 

 other transitional species. Thus the gap is yet large, and, considering 

 that Silurian rocks have afforded various embryonic forms in the 

 development of species of Trilobites, it is strange that nothing has 

 been found to illustrate the successive steps in the origin of the grand 

 sub-kingdom of Vertebrates. It is possible that further search may 

 be successful. 



In the Cretaceous formation of North America, leaves of plants of 

 modern type — the Angiosperms, like the Willow, Elm, Magnolia, 

 etc., and the Palms — occur, and exhibit a totally different character 

 in forest vegetation from that of the preceding period ; and the same 

 abrupt transition has been observed in Europe and other countries. A 

 long interval may have existed between the Jurassic and Cretaceous in 

 some regions, but hardly in all ; and if, after more complete investi- 

 gation, this distinction of the Jurassic and Cretaceous periods remain, 

 we may have to look to some other reason for this abrupt transition 

 than that from imperfect records. 



In the early Tertiary, the world, as the fossils show, was full of true 

 Mammals, related to the Tapirs and other kinds, many of great size ; 

 while no such Mammal has yet been detected in any earlier beds. 

 It is undoubtedly true that the break in the records, with regard to the 

 era preceding the Tertiary, is great ; but this fact does not supply all 

 that Science needs for a perfectly confident explanation of the break in 

 the system of Mammalian life. In the coal-bearing formation over- 

 lying the Cretaceous, in the Rocky Mountain region, there are the 

 bones of Dinosaurs ; while in the Eocene beds, resting on these, there 

 are remains of a wonderful variety of Mammals, some of elephantine 

 size. Probably a long time intervened between the eras of the coal- 

 beds and of the Tertiary bone-beds. But however long the time that 

 may be claimed, the abruptness of the transition is astounding, and 

 needs facts for its full elucidation. The same abruptness in the intro- 

 duction of the Tertiary Mammals occurs in the beds of other conti- 



